Radio frequency (RF) equipment uses a variety of approaches and structures for receiving and transmitting radio waves in selected frequency bands. Typically, filtering structures are used to maintain proper communication using frequencies assigned to a particular band. The type of filtering structure used often depends upon the intended use and the specifications for the radio equipment. For example, bandpass filters formed with cavity resonators are often used for filtering electromagnetic energy in certain frequency bands, such as those used for cellular and PCS communications. A bandpass filter allows only a predetermined band of frequencies to pass through a signal path.
Many cellular telephone applications require the filter to have a very low insertion loss such as 0.5 dB within a bandwidth such as 840 MHz to 870 MHz. Such a low insertion loss requires the use of cavity resonators that have a very high Q or quality factor. Q provides a figure of merit for a resonator system. However, such a filter requires relatively large cavities, which creates a relatively large distance between resonators positioned in adjacent cavities. The difficulty is that increasing the distance between adjacent resonators reduces the coupling between them and causes inefficient bandwidth.
Another problem relates to the effects of temperature on the resonate frequency of the filter and hence on the performance of the filter. As the temperature of the resonator increase or decrease, they will expand or contract accordingly, which will change the resonate frequency of the resonators. Such a change in the resonate frequency may effect the passband of the filter and the integrity of the signal being passed through the filter.
Accordingly, there is a need for a mechanism to provide an adequate amount of coupling between adjacent resonators. There is also a need for a cavity resonator that has a mechanism for improving bandwidth characteristics. There is a further need for an improved cavity resonator that filters out the 3d harmonic from a signal. There is yet a further need for a cavity resonator filter that compensates for the effect of temperature changes. There is a related need for a filter structure that has minimal changes in the resonate frequency due to temperature changes.